Electronic paper display devices (also referred to as electronic paper, digital paper) possess both characteristics of ordinary paper in displaying information and characteristics of computer display panels. The electronic paper display devices may be fabricated to have a thickness equivalent to that of a sheet of ordinary paper and can be utilized repeatedly; and moreover, the electronic paper display devices are capable of displaying dynamic pictures. Therefore, the electronic paper display devices are deemed as a kind of display means which is more environmentally friendly and expected to replace existing paper documents in the near future.
Most of the current researches by researchers focus on electronic paper display devices based on the electrophoretic technique. Electrophoresis refers to the movement of charged particles in an applied electric field. Electrophoretic display is a kind of bistable display, and particles in an electrophoretic liquid have two stable states. If there is no external force, these stable states will not be changed, and thus a display pattern can be maintained for a long time. Therefore, electrophoretic display needs an extremely low level of power consumption.
The research on an electrophoresis mode electronic paper display device has already begun as early as the 70s of the last century. In order to solve the problems of poor stability, short lifetime, etc., by realizing a micro-encapsulation treatment with electrophoretic particles, researchers of the Massachusetts Institute of Technology in the United States solved the problem of natural coagulation of particles, and stability of electrophoretic display was improved greatly. Industrialization of microencapsulated electronic ink has been successfully realized by “E-INK” company at present. Some companies have launched e-book products based on the microencapsulated electronic ink technology. Another method to improve stability of an electrophoretic solution has been proposed by “Sipix” company of the United States, which is a micro-cup technology. With the micro-cup technology, lattices with banks are firstly produced on a substrate, and then an electrophoretic solution is injected into the lattices, so that particles for electrophoretic display are obstructed from being coagulated by isolation banks between the lattices. Thus, stability and lifetime of the electrophoretic display are improved.
Regardless of the microcapsule technology or the micro-cup technology, in order to realize display of text information, the use of an active matrix drive technology is required. For example, a thin film transistor (TFT) technology is one way to achieve an active matrix.
An electrophoresis mode electronic paper display device usually comprises an upper substrate and a lower substrate disposed opposite to each other, and a layer of electrophoretic particles is filled between them. On the upper substrate is provided a common electrode; and on the lower substrate are provided pixel electrodes, which are usually arranged in a matrix shape and each of which corresponds to one pixel. With a TFT active matrix as an example, a pixel electrode is connected to a drain electrode of a TFT through a via hole, a source electrode of the TFT is connected to a signal line, and a gate electrode of the TFT is connected to a gate line. Based on the above configuration, switching-on of individual pixels is controlled by gate lines. When an ON voltage is applied to the gate electrode of the TFT, an active layer becomes conductive to make the drain electrode and the source electrode communicated with each other, and then a signal voltage can be applied to the pixel electrode, which is connected to the TFT, through the signal line. The signal voltage applied to the pixel electrode coordinates with a common voltage on the common electrode to form an electric field in the layer of electrophoretic particles, which causes electrophoretic particles to move so as to form a required pattern.
However, with respect to the electrophoresis mode electronic paper display device employing the active matrix driving technology, because the electrophoretic particles need to swim in a liquid and the swimming speed is relatively slow, the response speed of the electrophoresis mode electronic paper display device is relatively slow and it is hard to realize multiple-grayscale display. In addition to the above reason, with respect to an active substrate, there are the following two reasons. On the one hand, owing to a larger leakage current of the electronic paper display device, large storage capacitance is needed to guarantee good voltage stability, but large storage capacitance leads to a relatively long charging/discharging period, and this factor results in a relative slow refresh rate of the electronic paper display device. On the other hand, the driving mode of electronic paper is a pulse width modulation (PWM) driving method, i.e., one grayscale change can only become realized through more than ten or even dozens of refresh operations, and this further brings about the results that it is relatively difficult to realize the multiple-grayscale display of the electronic paper display device, image refresh rate is slow, and it is difficult to realize dynamic display. Furthermore, with the current drive mode, one update of image is completed by combining more than ten or even dozens of screen refresh operations together, and this combination of more than ten or even dozens of refresh operations is called as a pulse combination (waveform). Different grayscales generally need dozens of pulse combinations. Because the frequency for screen refresh is fixed (about 50 Hz), the frequency at which each pixel voltage is refreshed once is also fixed (i.e., consistent with the frequency of screen refresh), namely the intervals between pulses are fixed. Moreover, the pixel voltage for each refresh operation is also fixed (such as 0V, 15V or 30V). Therefore, possible combinations are less, and more grayscale displays may not be realized.